Sizing boiler allowing for future expansion

I'll be installing a new boiler soon in my 1835 2,000 sq ft farmhouse in CT. House has another uninsulated two story sheep barn attached to kitchen which I plan to finish a few years down the road. (about another 500 sq ft). I'm trying to weigh my options in considering sizing the new boiler which needs to be replaced soon. On the one hand, I'm trying to gain efficiency now, but by planning for the future expansion project, I want to make sure I have enough in reserve without oversizing too much.

Based on some online heat loss calculators, I've come up with a figure of around 75,000. So, I'm looking at the Burnham MPO IQ115 at 85,000 BTU, or going one size higher to the MPO 147 with 112,000, and adding the Alliance 35 gallon indirect HW heater.

I've spoken to a couple of installers over the phone, met with another, and all have said based on sq feet and length of baseboard I need at least 125,000-140,000 btu's, just for my current needs????

Figured I'd check in here to get some advice.

It's an old house, so it's pretty loose. Was insulated during a renovation in the 60's where PO's tore out all plaster, insulated all outside walls with R11 rolls, and re-sheetrocked. Saw some of it when I replaced some clapboards a few years ago. Not too bad. I actually found corncobs in the attic soffits too. Also insulated the ceillings in 2nd floor under the floor of an uninsulated walk-up attic. Unused Center Chimney with three fireplaces, all sealed with r16 glued to plywood jammed up into the flue. Fieldstone foundation and dirt basement. 17 windows, all with storms, 6 original single pane still original to the house with wavy glass, the other 11 replaced during the 60's renovation. Two zones, 100 ft of baseboard on the upstairs zone, and 80 ft downstairs.

If the house is reasonably air tight, with at least code-min double-panes (or reasonably tight older windows + triple-track storms) it's unlikely that your 2000' farmhouse has a heat load of 75K. If it IS that high due to air leakage or degraded batts, it's likely that it can be brought under 50K, maybe even under 40K cost-effectively with some air sealing and spot-insulation upgrades.

125-140K is and INSANE value for 2000 square feet of living space- even a reasonably tight tent could come in under that at 0F, and your 99% outside design temp is a few degrees warmer than that.

It's worth air-sealing and insulating the foundation with 2" of closed cell spray foam, and putting down a ground vapor retarder on the floor. If you plan to continue to heat with oil (sounds like you do), it's also worth putting down at least R5-R7.5 of rigid foam on the floor and pouring at least a rat-slab over it. (2" of non-structural concrete is enough), but basement floor insulation is not nearly as critical as the air-sealing and insulating the foundation to at least a couple feet below grade.

On clapboard sided building it's possible to blow cellulose in over existing batts on exterior walls, which will fill in the gaps and tighten up the place considerably. Stud spacing & depths in 1835 literally NEVER conform to 20th century batt sizes, so it's highly unlikely the R11 batts are performing better than R8-R9 as-installed. As long as there is at least some plank-sheathing under the clapboards a cellulose blow from the exterior is pretty low-risk/high-reward on oil use. Cellulose installers can either pop a few clapboards and drill the sheathing to execute the install or drill through it all & pound in a wooden plug to be trimmed flush with the siding prior to painting. Cosmetically the former looks better, but it's less labor to just drill & fill.

If you have a zip code and a mid or late winter oil bill with a K-factor stamped on it, it's pretty easy to calculate an upper bound on the design condition heat load based on the DOE efficiency on the old beast.

The length of the existing baseboard has no bearing on the actual heat load, but having excess limits short-cycling on the boiler. Having "too much" isn't really possible, but it affects the near-boiler plumbing, since the return water coming back needs to stay above 140F where it enters the boiler. There are various methods for achieving this.

The heat load of the additional 500 square feet is within your control. Insulating the walls with either blown cellulose or spray foam insulation is much tighter than batts, and going with better-than code new windows (or low-E storms over antique windows) makes a difference. And insulating/air-sealing the foundation is far more effective than trying to air-seal and insulate the floor. It won't break the bank to get the heat load of that addition down to under 20BTU/square foot (10KBTU), or even under 15BTU/ft (7.5K), given that it has at least one wall in common with the rest of the house.

Bottom line, don't go any bigger than the smallest boiler in the lineup- at $4+ oil you can't afford the inefficiency incurred by even further oversizing, but you CAN afford to fix all of the lower-hanging fruit on the heat load. Air-sealing is the most-critical, followed by insulation. Not all air-leakage counts the same- sealing the foundation and the upper-floor-ceiling/attic-floor interface are the most-critical, along with any plumbing & electrical chases or flues/chimney-chases that extend from basement to attic, since that quells stack-effect leakage that runs 24/365, and runs ever-faster with decreasing outdoor temps. Balloon framed walls can act as stack-effect flues too, so it's important to have some air-retardency to the cavity insulation. R11 batts too low-density- little more than an air-filter in the wall cavity from an infiltration point of view, but putting even low-density cellulose in the cavities reduces that by 90% or more.

It occurs to me that depending on how much radiation you put in the new addition it'll short-cycle the boiler for LOUSY efficiency. It's probably worth using a "reverse-indirect" as a central buffer to buffer the entire heating system and provide the domestic hot water function and slaving the boiler to the buffer's aquastat as it's only "zone". You can bump the temp of the tank up to whatever it needs to be for the baseboard to deliver enough heat in winter, (and drop it back to 140F for the summer. The zone controller for your heating zones can just activate the pump(s) &/or zone valves pulling water from the tank, and the boiler loop pump can be controlled by the tank's aquastat. It's approximately (but not exactly) this:

There are a few vendors of this type of indirect, ThermoMax (<<they may have moved out of that product line- couldn't find 'em on the web), Ergomax, Everhot (EA series only), etc. They're a few hundred more expensive than other indirects because the size of the internal heat exchanger needs to be a bit bigger, but you can't dump 75,000BTU/hr of boiler input into a 500' zone with 1/10th the heat load without either RIDICULOUSLY oversized radiation for the zone or a serious short-cycling problem, which wears out the boiler and ruins it's as-used operating efficiency. Even a ~25 gallon version makes a significant difference on both the number of ignition cycles and the length of burns.

While ErgoMax has ports for all connections built into the tank, the EverHot EA (and ThermoMax, if you find them) will need tees placed close to the tank to achieve a similar configuration. (Heating pros might recognize that this is basically a primary/secondary configuration, where the reverse indirect tank is just a fat & massive hydraulic separator. The twist is that boiler loop need on run on every zone call, and can be slaved to the tank's aquastat.)

If you don't go with the buffering-indirect approach it will be necessary to add mass when you add the new zone, which can be done with a cheap electric HW heater (not wired up). To avoid a huge slug of too-cold water stressing the boiler it's best if it's plumbed similarly to the upper diagram, but with the indirect being a separate zone not running off the buffer. If the tank is not maintained at temp, there needs to be separate return-water temp protection plumbed in.

Dana - Thanks for the detailed response, I'm still digesting it. I replied with some more info yesterday, but apparently it didn't go through, so I'm trying again.

Zip is 06524. Can't find K factor, but looked back through five years of bills, and winter months averaged between 120-180 gal/month of oil. Totals ranged from 790 gallons to 1140 gallons for the year during that time. Old boiler is rated at 129,000 btu

House is Timber framed, and 3/4 of it has no sheathing beneath the clapboard. It goes clapboard/insulation/sheetrock. The other 1/4 is a circa 1900 addition that has sheathing beneath the clapboards.

Basement clean-up and tightening up is definitely on teh short list of things to do. Thanks for the all the great advice about what to focus on there.

For Bethany it's safe to use Hartford's 99% outside design temp of +6F. It might be slightly cooler than that, but it's probably a few degrees warmer than Waterbury (design temp= +2F).

If we assume a 1000gallon/year as an average, and Hartford's ~6150 annual heating degree days , and assuming your boiler is running at 85% efficiency (it's almost certainly less, since it's oversized and aged) the simple math breaks down this way:

1000 gallons/6150 HDD is 0.16 gallons per HDD

There are 138,000BTU/gallon so that's a source-fuel use of 0.16 x 138,000= ~22,000 source-BTU/HDD.

But in an 85% burner only 85% of that heat ended up in the heating system water, so that's 18,700 BTU/HDD.

There are 24 hours in a day, so per degree-hour that's a heat load of 18,700/24= 780 BTU per degree-hour

... all using base 65F as the heating/cooling balance point (pretty close for most houses).

Using a design temp of +5F, that's 65F-5F= 60 heating-degrees

so with a maximum of 780 BTU/degree- hour, that means your heat load at + 5F is less than or equal to 60 x 780= 78,000BTU/hr as an absolute max, which is pretty close to your online heat load numbers, but in reality, a significant over-estimate, especially if you're heating your hot water with the boiler, and the fact that it's roughly 2x oversized.

If you're not heating water with the boiler (and letting it go cold for the warmer 4 months) you're probably looking at 75% as-used efficiency and the heat load is actually (75%/85%) x 78,000= ~69,000 BTU/hr.

If you ARE heating water with the boiler, at least 20% of the fuel use is for heating hot water, so the raw calc for just the space heating load is only 80% of that:

0.8 x 78,000= ~62,500BTU/hr

But since the boiler is still ~2x oversized and old, figure it's running 75% not 85%, which cuts the heat load number to:

(75/85) * 62,500= ~55,000BTU/hr.

These are VERY realistic numbers for that level of fuel use for an air-leaky somewhat insulated house that size, and depending on the actual condition of the boiler the heat load could be even lower.

If you want further verification of where your as-used efficiency & heat load there's a reasonable boiler-modeling tool that can take either gallons/year or K-factor as an input, that was developed at Brookhaven Nat'l Labs, that's downloadable here. Scroll down to the links for the FSA Calculator and it's user manual. It's not bug-free, and isn't very flexible on design temps (it seems to use numbers closer to the absolute lows than the current 99% weather data numbers), but delivers fairly accurate results, with bit of interpretation.

Based on your fuel use numbers and the fact that you can probably knock off 20-25% cost effectively with air sealing and some insulation upgrades, even allowing 10,000 BTU/hr for the addition you'd safe going with the MPO IQ115.

If oil prices don't drop dramatically by the time you build it out, it may be cheaper/easier to heat the addition with a 3/4 ton or 1-ton ductless mini-split rather than extending the hydronic system, which would also have the benefit of high-efficiency air conditioning. These things have gotten INSANELY more efficient over the past 15 years, and the better ones still put out a lot of heat even at -20C/-4F. (The Mitsubishi MSZ-FE series still put out full-rated heat at +5F.). The seasonal average coefficient of performance (COP) of these will be about 2.7 in this climate, maybe a bit more, but for the sake of the financial argument lets assume it's only 2.5, and that electricity costs $0.20/kwh (10% more than the current CT average residential retail rate.)

At a COP of 1 you get 3412BTU/kwh, so for a COP of 2.5 you get 2.5 x 3412= 8530 BTU/kwh

Say the new oil-boiler really delivers 87%, that means for each gallon you get 0.87 x 138,000= 120,000BTU/gallon

So for comparison's sake, it takes 120,000/8530= ~14 kwh of power use to equal one oil-gallon's worth of heat.

At 20cent/kwh, that's like heating with $0.20 x 14= $2.80/gallon oil.

Seen oil that cheap lately? (Me neither, but it's been less than a decade...)

A mini-split heat pump isn't super cheap, but you can get a decent 1-ton for under $4K/installed, and a 3/4 ton for under $3.5K/installed. (I had a 1.5 ton Mitsubishi installed at my mother's place last winter for under $4.5K, and she got a $1K kick-back from the utility. A buddy of mine installed three 1.5-ton units to heat/cool a 3-family building he rehabbed in Worcester MA at about $4.1K/per-each) A 3/4 ton unit is rated 9000BTU/hr for cooling but will usually put out over 10,000BTU/hr at +5F. The Mitsubishi MSZ/MUZ-FE09 is rated for 10,900BTU heating @ +5F, and is still putting out over 7500BTU/hr @ -13F, but there are Fujitsu and Daikin 3/4 ton units that are as-good at +5F. You can buy a 3/4 ton high-efficiency Mitsubishi or Fujitsu for under $2K online, but it's better to let a qualified tech install it unless you're really into it. Look for heating HSPF ratings of over 9, and cooling SEER over 16.

The circa 1900 addition with sheathing under the clapboards can be safely blown tight with cellulose, but the older part that would likely create some issues, unless the roof has big overhangs to prevent rain penetration behind the clapboards. Cellulose would wick-up rain penetration and store it, causing the paint to fail, and possibly creating rot conditions at the clabboards, since the back sides are likely unpainted, and need to dry into the cavity. Without a site-inspection it would be tough to say for sure what the right approach would be. (A deep energy retrofit, pulling or foaming over the old siding and adding sheathing cavity-insulation and foam insulation under the new siding probably isn't in the cards unless subsidized heavily.)

You can probably get away with blowing in new-school fiberglass at 1lb density to fill in any gaps & voids on the un-sheathed sections, but tightening up the air leakage will be most-safely done on the interior side of the wall, not by dense-packing the cavities. But going at every plumbing & electrical penetration with caulking & 1-part foam is worth it, as is spray-foaming the interior side of the foundation sills & band joists with 2" closed cell foam. Fix any of the mortar issues with the fieldstone first, but 2lb polyurethane does a GREAT job of sealing & insulating. In a heated home there is zero risk to the foundation, and it in fact lowers the risk of frost-heaving by keeping the basement warmer near the base. (In unheated barns, etc insulating the foundation requires some amount of exterior treatment or it can frost heave.)

Not long winded...all great and appreciated information. I'm the type of person who knows a little about a whole lot of things, and I am always looking to learn more.

The one question I have is why does anyone I call first ask if I know how many feet of baseboard I have? I've had four different people ask me that and say that is the starting point. If the house warms up and stays comfortable with the existing baseboard heat, then they reason that there is some kind of formula they use to determine the minimum size boiler that can "handle" the length I have. None of their estimates come in less than 120,000 btu minumum with my 180' ft of 3/5 slant fin baseboard?????

Heating contractors should ask about baseboard for a different reason. It makes no sense to install more boiler than than the baseboard can get rid of. The heat load you did "online" is still closer than the "professionals", which is an embarrassment to the industry and to me personally.

I would give the job to the first guy that comes up with a sample heat load.

As for the Ergomax. It is simply a tankless coil. With a cast iron boiler the short cycles are likely not that short. The new addition should include panel radiators with individual control driven by the main floor thermostat.

The Ergomax is not efficient at storing hot water or making domestic hot water. It is a good hybrid for wood boilers perhaps. The problem is with the minimum temperatures necessary with any tankless coil. I would rather have outdoor reset.

The "tankless coil" in the Ergomax has several times the heat exchange capacity of an embedded coil in a boiler, and with 26 gallons of water (the smallest version), it has many times the thermal mass of any triple pass oil boiler that would work here. It's not some 5 gallon sidearm type heat exchanger, it's truly a "tank", and therefore "tankless coil" would not be a very good description.

Make no mistake, it's not tankless at all, it's precisely a buffer tank, but one with heat exchanger for domestic hot water. Whether it's the right solution here remains to be seen, but if the future 500' addition is going to be a separate & small hydronic zone, it might be, since it's cheaper than a separately zoned indirect + a buffer tank.

The existing smaller zone with 80' of fin tube puts out ~48,000BTU/hr at 180F AWT, and the MPO IQ115 is putting out 75,000 BTU/hr, so it's about 27,000BTU/hr (= 450BTU/min) of excess. The MPO IQ115 has only 7.7 gallons or ~65lbs of water, the 80' of fin tube and distrbution plumbing might have another 5-6 gallons call it 50lbs, figure another 5 lbs of water-equivalent thermal mass in the plumbing & boiler-iron you're looking at only 120lbs of water-equivalent thermal mass. With 450 BTU/min in to 120lbs of water the temp rises 3.75F/minute. If the boiler smart-controls (or can be programmed with) a hysteresis of 20 degrees or more around it's outdoor reset, that's a min-burn time on the order of 5 minutes, which is probably fine. It's a smart enough controller that I'm sure you can get there, and the hysteresis can probably be stretched even further and still have enough heat into the zone. But going 1/4 or even half that amount of heat emitter on the new expansion zone you'd run out of hysteresis-room on the zone mass.

Stick a 26 gallon buffer (like an ErgoMax or something else) in the middle, and all of those concerns goes away,since it adds over 200lbs of water mass to the system, and it's standby losses are about the same as any other indirect.

The odds that a much better sealed and insulated addition would have similar enough heat loss characteristics that it would balance reasonably if heated with an extension of either of the existing zones is pretty remote.

BTW: With 180' of baseboard and a design heat load under of 75K(and probably closer to 60K), that's 320-420BTU/ft, which means it can probably deliver the heat at design day with 140-150F water. Since the MPO-IQ115 has internal low-temperature protection & control and outdoor reset you can probably run the system at domestic hot water temperatures, most of the time, which will be extremely efficient compared to running it at 180F or 220F or wherever the current beastie-boiler runs it.

Most fin-tube puts out about 600BTU/foot when the average water temp is 180F, 700BTU/ft with an AWT of 200F. So if the boiler's high-limit is set to something like 210-215F and you get a 20-30F delta across the fin-tube for a 200F AWT, the fin-tube balances with the boiler's output at (180' x 700BTU/ft =) 126,000 BTU/hr.

So with a high-limit over 200F your old beastie-boiler probably doesn't short-cycle with both zones running, and if it has enough internal mass (it's probably at least 2x the thermal mass of the MPO-IQ115) and that if the low limit set to 140F or 150F it probably wouldn't short cycle on a single zone call either.

BTW: Looking at the spec, the DOE output of the MPO-IQ115 is 98,000, not 85,000, (and not the 75K number I blinding inserted into the analysis in the last post), and would be nearly 2x oversized for the fuel-use estimated heat load. The 85K number is an I=B=R number which is the rating if the boiler is located well outside of conditioned space, like in another building or something. If the boiler room is in the basement and you don't have insulation between the first floor and the boiler room, use the DOE number. It looks to me like you should instead be looking at the MPQ-IQ84, which has 74K of DOE output, which is still plenty, given that the fuel use estimate heat load number is probably at least 15-20% to the high side. The 78K it's a true upper-bound type of number, unless you normally let the place cool off to 50-55F on overnight setbacks or something.

If you've been heating domestic hot water with the old boiler (with either an indirect or an embedded tankless coil) you're really looking at no more than 62K for a heat load @ +5F, and it's probably under 60K. Even though it does get below +5F at least 1-2x most winters, even if you sized it exactly at the heat load at the 99% number you won't be cold. It has to stay below +5F for hours to lose much ground, and in a 25 year record cold snap you got up at dawn one morning and it was only 60-65F, it would still catch up by 9AM as the outdoor temp rose. With 74K of DOE output (the -IQ84) and a ~60K heat load at +5F you have quite a bit of margin- it would still keep up even at -10F which doesn't happen very often in Bethany. (The record low in Weatherspark's database is -11F.) With 98K of output (the -IQ115) you'd be good down to about -35F which probably hasn't occurred in the past millenium.

The guys coming up with 125K+ for a heat load are simply nuts. For a 2000' house that's over 60BTU/foot of living space, which would be high even for a house with zero insulation and no storm windows over the wavy-glass antiques. But I s'pose if you left a window or two open it could get that high. Most crusty southern NE old schoolers would use a 35-40BTU/foot rule-of thumb estimate for a 2000' antique house, (which would still reliably overestimate, usually by 50% or more) and at 40BTU/ft that's still only 80K. Maybe they were unduly influenced by the size of the old boiler and the 180' of baseboard.

The Buderus boilers are something of the gold-standard- with about the same burner output and similar internal low return-temp protection, it would run similar temps as the MPQs.

"Maybe they were unduly influenced by the size of the old boiler and the 180' of baseboard."

OK, so I asked why the need for a 125k+ boiler, and he said 600 btu/ft x 180 gives the jumping-off point (108,000) He also said it will provide for quicker recovery for the new indirect. I have currently been running a tankles coil in the old boiler.

The capacity of the baseboard and the quantity of heat required are often NOT anywhere close to being the same. More baseboard may make the room more comfortable by providing a more even heat, but often, you may not need that much. Buying a bigger boiler with its likelyhood of short-cycling on most loads, is a very poor tradeoff. If you actually need more hot water, buy a bigger tank, don't buy a bigger boiler! I can take an endless shower with my 60K boiler because I have a significant indirect, big enough to quickly fill a large soaking/air tub. A good indirect has very little in standby losses (some of the better ones come in at 1/4-degree per hour or less), so heating up a big slug of it, other than the cost and space for it, isn't worth it to go too small and compensate with a bigger boiler. A typical gas stand-alone WH is likely in the range of 40-50KBTU (but much lower in efficiency than a good boiler). Most indirects are setup as a priority zone, and if you do need it, ALL of the heat goes into reheating that tank, so since the boiler is often larger (and more efficient), you can usually get by with a smaller indirect. But, a larger one means longer boiler run times (if it lets you adjust the trip point to reheat) which ends up more efficient.

On the coldest design day, the boiler should be running constantly, and if you get a mod-con, it can adjust the output to match the need on more days of the heating season. Since you have more mild days, with a boiler probably 3-4x what your design day needs are, even if it can modulate, it will by cycling off/on into an early grave.

Living space is more comfortable (and more ecconomical) if you can use say 120-degree water running constantly than running 180-degree water periodically. And, there's less expansion/contraction noises associated with it as well.

The dealer you are talking to is using old-school methods that guarantee an oversized system. That wasn't too bad when fuel was cheap, but it still took a hit on efficiency and never provided all of the comfort it could have if designed properly.

A tankless system requires a huge boiler to provide lots of hot water for showers, etc. With a tank, you have like all day to reheat it for the morning showers (but it will recover quickly, regardless)...it's like day and night - you have no flow restrictions, and if sized properly, you should never run out.

As has been said...you need a good heat load analysis...this takes into account the windows, doors, insulation, site plan, location and some other factors. Your house is probably in the 25-30KBTU range of need on the coldest normal day of the winter. Picking a boiler based on length of your baseboards is just totally wrong. It's like buying a 500Hp motor for your minivan...it will never get used, cost a fortune to keep filling the tank, and probably not be that great an around-town vehicle. Now, maybe if you lived in Germany, had 5 children, and ran the Authobahn regularly, but who can afford that at $8+ per gallon fuel there?!

Get a good manual J heat load analysis done, even if you have to pay someone to do it so you know what is truly needed...then, design the system to provide that.

At the risk of also getting unduly hung up on the length of the baseboard, I decided to peek under the covers, and measure the actual length of "Finned" baseboard throughout the house. Downstairs I knocked off 7 ft, but upstairs, I knocked 39' off my total. Most rooms had finned pipe under only half of the covers. For the most part, there was still 3/4 copper running the length of the wall to the next room when it wasn't covered in fins, but I also knocked off about 5 feet, where pipe came up through the floor, and the cover was a true "dummy" where a couple of feet had no pipe under the cover. So my original # of 180' only includes 134' of fin tube, plus 41' of plain 3/4" copper.

Unless the boiler was running full blast on the coldest day and never shutting down to keep the house warm, it is oversized! Dana has explained it numerous times, and you can read them if you search...a good way to see how much heat you actually used is to take your fuel use and match it up with the degree-day history files for your zipcode. You should have access to your fuel bills, and you can download the degree-day data for the period. That will tell you how much energy you actually put into the house for that period. You'd want to do that for a full heating season, and if you have the data, you could do it for several to get an average, since not all winters are the same. A little margin wouldn't hurt, but you don't want much. If you use huge setbacks overnight, the extra margin will help recover faster, but once you achieve the setpoint, that extra margin costs money in both efficiency and life of the unit.

"Maybe they were unduly influenced by the size of the old boiler and the 180' of baseboard."

OK, so I asked why the need for a 125k+ boiler, and he said 600 btu/ft x 180 gives the jumping-off point (108,000) He also said it will provide for quicker recovery for the new indirect. I have currently been running a tankles coil in the old boiler.

Click to expand...

Downright LOUSY reasons all around!

Size the indirect for the hot water heating load (sizing it for largest tub-fill is about right), and run it as a "priority zone". With 74K of boiler output you can run a 2gpm shower FOREVER, (that's more than 2x the burner output of a typical 40 gallon gas-fired standalone tank!) but if you have a standard-sized tub that you want to fill at 8-10gpm you'll want at least 30 gallons of tank,i f kept at 140F. You can go somewhat smaller if using high storage temps. (Using a 26 gallon ErgoMax and "sharing" output with radiation you'd want to run it higher than 140F during the heating season.)

The length of the baseboard determines the water temp required to deliver the load at the design temperature, which is going to be well-under 180F, even with the updated 134' fin-tube length. The fact that you've been running a tankless coil, at your annual fuel use it means that your space heating load at the 99% design temp of about +5F is well under 70K, and could easily be under 60K. So it's likely that the highest temp you'd ever need on the baseboard is about 155F, and if you insulate & air seal you can probably bring it down to 145F at the outside design condition.

With outdoor reset control on the boiler the average will be much lower. By running the baseboard at lower temp means the distribution & standby losses to the semi-conditioned basement will be lower, and the system-efficiency much higher.

On an old-school boiler you'd have to add in some near-boiler plumbing to protect the boiler (and the flue) from acidic condensation, but in the MPO-IQ84 that protection is internal to the boiler, no extra system design necessary on that front.

DO run the FSA calculator using your fuel use and the fact that it's an embedded coil (==massive shoulder & summer season standby losses) and let the tool auto-adjust your heat load estimate. (When I run it using your boiler size using Hartford with an older design temp of -1F, even plugging in 1300 gallons/year it's coming up with heat loads under 50K, which is still credible.).

" On the coldest design day, the boiler should be running constantly..."

and

" Unless the boiler was running full blast on the coldest day and never shutting down to keep the house warm, it is oversized"

With a non-modulating boiler sized EXACTLY to the heat load at design temp it would have burns that may go for over an hour or two on the coldest MORNING of the year (while still keeping you warm), but as the day grows warmer it'll still cycle off for extended periods, since the heat load falls below the boiler's output with rising outdoor temps, lowered even further by solar gains. If it runs constantly

With a modulating boiler it should be running pretty much constantly for the entire month of January if sized exactly to the heat load, since the heat load even at the daily highs will usually be within the modulating range of a modulating-condensing boiler. But with fixed output boiler (like 99% of all oil-fired boilers) it'll be cycling, even on the coldest day of the year, but the burns will be several 10s of minutes long on those days in a decently designed system if sized perfectly.

In practice you can only go so low for burner output with oil as a fuel, and even the smallest burners are usually 2x oversized for new code-min houses, if only 1.5x oversized for a leaky antique house.

You're existing boiler is probably getting at-best 55% efficiency according to the FSA calculator, but an MPO-IQ84 would be hitting in the high-80s. Plugging in 1100 gallons/year and a Hartford climate, with an 129KBTU "old boiler w/ internal coil" it's estimating a heat load under 40K but an efficiency just under 55%. Swapping in the smaller high-tech boiler + indirect would cut your fuel use by a bit more than 1/3. Using the bigger -IQ115 with 98K of output would be a mistake, it would cost a bit more up front and every year there-after.

Keep in mind also that if you have mostly or all fin tube radiators that those are not the best case scenario when switching to a modcon. They're low mass and will not create a situation where you'll have the delta-t that you're looking for. Just another reason to size the unit for exactly what you need, no more.

I had a Burnham modcon installed two years ago. It's GROSSLY over sized for my house as it stands now. Luckily my unit can be dialed back so that I'm only using a portion of its heating capacity. My indirect tank is 50 gal and even for that priority I have the boiler dialed back. I've never run out of hot water.

The idea is to create a situation where the output water temp doesn't match the returning water temp. One that happens the water temp will just rise to the maximum limit set on the curve and the boiler will shut off until the water reaches the low end temp.

The mass of any oil-fired boiler is many times that of a gas-fired mod-con, and those with smart-controls will automatically exercise that mass with bigger hysteresis to keep burn lengths reasonable. But even the smallest & smartest oil boiler would have a problem if the planned addition is heated with low-mass radiation unless mass is added to the system, which is what prompts pondering the Ergomax configuration.

As currently zoned the current fin-tube lengths are manageable for the MPO-IQ084, if the boiler's controls fully utilize the available hysteresis range (which is considerable compared to dumb-controlled oil boiler that aren't inherently protected from cool return water), but with a low outdoor reset temp on the MPO-IQ115 the burn lengths will be shorter, average efficiency lower. The smallest zone has 62' of fin-tube into which the boiler is dumping 85,000BTU.

If the outdoor reset is trying to adjust it to 120F AWT (about as low as you'd want to go with fin-tube- output is less predictable below that), the fin-tube is only emitting ~200-220BTU/hr/ft, so x 62' it's ~13,000BTU/hr into the zone, with 72,000BTU/hr (1200BTU/min) of excess. With something on the order 100lbs of water-equivalent mass in the boiler + zone the water temp rises (1200/100)= 120F/minute- the burns will be SCREAMING short unless the water temps were allowed a great deal of hysteresis. Since the fin-tube puts out more heat at the higher temps you'd get at least 3 minutes of burn out of it, but probably not 5.

With the -IQ84's 74K of output you'd have ~60,000BTU/hr of excess (1000BTU/min) and the temps would rise at 100F/min, and the temps would still be pretty short. With an Ergomax (or any 25-30 gallon insulated tank) buffering the system it effectively triples the thermal mass, and triples the burn time at any temperature or hysteresis.

At 140F AWT you get ~330BTU/ft or (x 62'=) ~20,000BTU/hr of output from the fin-tube, which would still leave 65,000BTU of excess with the -IQ115, (almost 1100BTU/min) but only 55,000BTU/hr of excess (920BTU/min) out of the -IQ84 for a rise-rate of 92F/min un-buffered, 31F/min if buffered with 25-30 gallons.

At 180F AWT only half the output of the -IQ084 would be emitted into the zone with a T-rise rate of ~ 37F/min (unbuffered), and it would be well under half with the -IQ115, with a T-rise rate of ~80F/min (unbuffered), more than 2x the rate of the smaller boiler.

These are just a few points on the curve to get a feel for it rather than doing the formal integral calculus to come up with more precise burn times, but know that if the outdoor reset doesn't allow a very substantial hysteresis you're looking at very short burn lengths, in either case, but they'll be much shorter and lossier with the -IQ115 than with the -IQ084. Adding thermal mass would be of benefit with either version, but it would become essential with the bigger boiler, no matter how much hysteresis is programmed in, since the temp is still climbing fast at 180F. Getting 10+ minute minimum burn times would be ideal- even 5 minutes would be OK, but that would not be in the cards with the bigger boiler unless mass is added.